Py3: Replace six.string_types with str

In Python 3, six.string_types is just an alias for str.

See: https://pagure.io/freeipa/issue/7715
Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>

ipalib/config.py

@@ -253,7 +253,7 @@ class Env:
             )
         # pylint: enable=no-member
         assert not hasattr(self, key)
-        if isinstance(value, six.string_types):
+        if isinstance(value, str):
             value = value.strip()
             if isinstance(value, bytes):
                 value = value.decode('utf-8')